Aircraft brake systems can generally carry out both commanded and uncommanded braking. Commanded braking occurs when a pilot depresses a brake pedal or in some other manner produces a signal to initiate braking. Uncommanded braking occurs when an aircraft computer or aircraft brake control system (BCS) actuates an aircraft's brakes without pilot input. Uncommanded braking is used, for example, to test an aircraft's brakes prior to landing to ensure they are in working order. Uncommanded braking may also occur immediately after take off to stop the landing gear wheels from rotating before the landing gear is retracted.
Because uncommanded braking is controlled by a computer, there is a possibility that it will occur as the result of a software bug or malfunction. Such uncommanded braking during takeoff could be disastrous, and thus braking systems must be designed so that uncommanded braking at critical times such as takeoff is extremely improbable.
This can be accomplished by using extremely reliable software, satisfying DO178B level A standards of no more than a one-in-one-billion failure rate, for example. However, testing and maintaining software to this level of reliability is difficult and very expensive. It is preferable to use software satisfying the lower DO178B level B standard of reliability, or having no more than a one-in-one-million failure rate. Therefore, to achieve an acceptably high level of reliability using such software, a redundant brake control path has heretofore been used. This may comprise, for example, a hard-wired circuit between a brake pedal and a brake driver (a device that controls either the electromechanical actuator of an electric brake or the hydraulic controls of a hydraulic brake). Unless a signal is present on this circuit during critical times such as takeoff (e.g. when weight is sensed on the aircraft wheels), to indicate commanded braking, a computer command to actuate the aircraft brakes will be ignored.
This solution provides adequate results. However, wiring for the redundant circuit has to be run from brake pedals in the cockpit to a driver in or near the aircraft wheel well. This increases the weight and cost of the brake system and adds maintenance costs as well. It would therefore be desirable to provide a braking system for an aircraft that is controlled by software in a manner that makes uncommanded braking during takeoff extremely improbable but which does so without the use of DO178B level A software.